1. Field of the Invention
The present invention generally relates to the identification of integrated circuit chips by means of an identifier which differentiates one chip from another.
2. Discussion of the Related Art
FIG. 1 is a simplified top view of a wafer 1 of a semiconductor material on which circuits or chips 2, all identical, have been formed. Reference will be made to integrated circuit chips, knowing that each chip may comprise one or several active and/or passive circuit(s). Currently, a much higher number of chips than what is shown is formed on a same wafer. At the end of the manufacturing, chips 2 are separated by being cut, for example by means of a saw, along sawing paths 3 shown in dotted lines. In many applications, it is necessary to have a way to uniquely identify the chips originating from a same manufacturing batch. Such an identifier is, for example, used for recognition or ciphering purposes in devices such as payment cards.
An identification method comprises, for each chip, storing a number in a non-volatile memory of a circuit. This method requires a specific step, be it during the manufacturing (masked writing into a ROM) or after the manufacturing (electric or optical writing process).
Another group of methods comprises identifying a chip based on physical parameters with values linked to the manufacturing uncertainties. Electric parameters of the chip may especially be measured. For example, a threshold voltage of a transistor, a resistance, or a stray capacitance may be measured. Such features are sensitive to dispersions resulting from the inaccuracies of manufacturing technologies. It will be spoken hereafter of technological dispersions. Thus, it can be considered that the parameter(s) taken into account are specific to a chip and form a unique identifier. Currently, a large number of identification elements capable of providing, for each chip, a unique identifier, based on physical parameters linked to the manufacturing of these elements, is provided in the chips. The value differences forming the identifier of a chip are by nature very small. They range within the margin of inaccuracy linked to the manufacturing.
A disadvantage of using a measurement of electric parameters is that these parameters are likely to vary over time, during the lifetime of a circuit. Accordingly, the obtained identifier is not stable. Further, such features may be sensitive to parasitic dispersions especially depending on the temperature of use. A disadvantage of methods based on parameter measurements thus is that identifiers which may vary from one measurement to the next one are obtained.